A two-step path to inclusion formation of huntingtin peptides revealed by number and brightness analysis

Abstract

Protein aggregation is a hallmark of several neurodegenerative diseases including Huntington's disease. We describe the use of the recently developed number and brightness method (N&B) that uses confocal images to monitor aggregation of Huntingtin exon 1 protein (Httex1p) directly in living cells. N&B measures the molecular brightness of protein aggregates in the entire cell noninvasively based on intensity fluctuations at each pixel in an image. N&B applied to mutant Httex1p in living cells showed a two-step pathway leading to inclusion formation that is polyQ length dependent and involves four phases. An initial phase of monomer accumulation is followed by formation of small oligomers (5-15 proteins); as protein concentration increases, an inclusion is seeded and forms in the cytoplasm; the growing inclusion recruits most of the Httex1p and depletes the cell leaving only a low concentration of monomers. The behavior of Httex1p in COS-7 and ST14A cells is compared.

Figure 1

Determination of molecular brightness. A COS-7 cell transiently transfected with mEGFP is shown. (A) Average intensity image of a single optical plane with excitation at 488 nm at 0.8% laser power and 20 μs/pixel. (B) Selection of pixels whose brightness corresponds to monomers, B = 1.074 indicates that monomers are distributed uniformly throughout the cell. (C) Brightness histogram. Selection of pixels corresponding to the Gaussian distribution centered at B =1.074 cpm, (ɛ = 3700 cpsm) identifies the monomeric protein. Only a few pixels are outside the distribution of the monomeric brightness. These pixels tend to concentrate at the cell border or at the border of internal membranes. This analysis depicts the N&B map of a uniform, monomeric EGFP species.

Figure 2

COS-7 cell transiently transfected with Httex1p-25QP-EGFP. Cells were imaged 51 h after transfection with 0.8% laser power at 488 nm and 20 μs/pixel. (A) Intensity image of a single plane shows widespread presence of Httex1p-25QP-EGFP, slightly lower levels in the nucleus. (B) Selection of the pixels with brightness corresponding to the monomers (B = 1.074, ɛ = 3700 cpsm). (C) Pixel brightness histogram confirms the prevalence of monomers.

Figure 3

COS-7 cell transfected with Httex1p-46QP-EGFP. Cells were imaged with 0.8% laser power at 488 nm and 20 μs/pixel. (A) Average intensity image reveals widespread distribution. (B) Selection of the pixels with brightness corresponding to monomers (B = 1.074, ɛ = 3700 cpsm) detrended (first row) and without the detrend filter (second row). Monomers are distributed throughout the cell. (C) Selection of the pixels with brightness corresponding to oligomers (centered on decamer, B = 1.74, ɛ = 37,000 cpsm) detrended (first row) and not detrended (second row). (D) Brightness histogram with the distribution of the pixel after detrend (first row) and before detrend (second row). The fraction of higher brightness pixels has increased with respect to the histogram of D. In the case of not detrended data, the values of B recovered are larger because of the fluctuation due to the cell motion.

Figure 4

Time evolution of COS-7 cell transfected with Httex1p-97QP-EGFP. Cells were imaged with 0.8% laser power at 488 nm and 20 μs/pixel. The different rows correspond to the different phases. The first column shows the average intensity image. The molecular brightness first increases and then decreases after the formation of the inclusions (red arrows). The green arrows indicate the points of nucleation. The selection of pixels with the brightness of the monomers (B = 1.074, ɛ = 3700 cpsm) and oligomers (B = 1.74, ɛ = 37,000 cpsm) are reported in the second and third columns. In the fourth column the percentage of pixels corresponding to monomer (green) and oligomers (yellow) are reported, this graph has been obtained from the analysis of different cells at the different phases shown. The number of pixels with brightness corresponding to decamers increases until the inclusion is formed and recruits the majority of the protein.

Figure 5

Time evolution of ST14A cell transiently transfected with Httex1p-97QP-EGFP, cells were imaged with 0.8% laser power at 488 nm and 20 μs/pixel. (First column) Intensity image, the green arrow indicates a nucleation point; red arrows indicate the inclusions. (Second column) Selection of pixels with the brightness of the monomers (B = 1.074, ɛ = 3700 cpsm). (Third column) Selection of oligomers (centered on decamer, B = 1.74, ɛ = 37,000 cpsm). In the fourth column the percentage of pixels with brightness corresponding to monomer (green) and oligomer (yellow) in different cells in the four phases are reported. The percentage of oligomers increases until the inclusion is formed and recruits the majority of the protein.

Figure 6

Model of Httex1p-97QP-EGFP aggregation. Phase 1, initial expression of Httex1p. The protein concentration is low (<1 μM), only monomers are detected. Phase 2, at higher protein concentration (∼1 μM of monomers), small oligomers (5–15 proteins) form in a process that lasts few hours. Phase 3, inclusions start to grow at each nucleation site. Phase 4, the inclusions recruit most of the protein present in the cell leaving only monomers at low concentration.